Binarity in carbon-enhanced metal-poor stars Starkenburg, Else; Shetrone, Matthew D; McConnachie, Alan W ...
Monthly Notices of the Royal Astronomical Society,
06/2014, Letnik:
441, Številka:
2
Journal Article
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A substantial fraction of the lowest metallicity stars show very high enhancements in carbon. It is debated whether these enhancements reflect the stars’ birth composition, or if their atmospheres ...were subsequently polluted, most likely by accretion from an asymptotic giant branch binary companion. Here we investigate and compare the binary properties of three carbon-enhanced subclasses: The metal-poor CEMP-s stars that are additionally enhanced in barium; the higher metallicity (sg)CH- and Ba II stars also enhanced in barium; and the metal-poor CEMP-no stars, not enhanced in barium. Through comparison with simulations, we demonstrate that all barium-enhanced populations are best represented by a ∼100 per cent binary fraction with a shorter period distribution of at maximum ∼20 000 d. This result greatly strengthens the hypothesis that a similar binary mass transfer origin is responsible for their chemical patterns. For the CEMP-no group we present new radial velocity data from the Hobby–Eberly Telescope for 15 stars to supplement the scarce literature data. Two of these stars show indisputable signatures of binarity. The complete CEMP-no data set is clearly inconsistent with the binary properties of the CEMP-s class, thereby strongly indicating a different physical origin of their carbon enhancements. The CEMP-no binary fraction is still poorly constrained, but the population resembles more the binary properties in the solar neighbourhood.
Abstract
The updated
H
-band spectral-line list (from
λ
15000–17000) adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) for the SDSS-IV Data Release 16 (DR16) is presented ...in this work. The APOGEE line list is a combination of atomic and molecular lines, with data drawn from laboratory, theoretical, and astrophysical sources. Oscillator strengths and damping constants are adjusted using high signal-to-noise, high-resolution spectra of the Sun, and
α
Boo (Arcturus), as “standard stars.” Updates to the DR16 line list, as compared to the previous DR14 version, include the addition of molecular H
2
O and FeH lines, as well as a much larger (by a factor of ∼4) atomic line list, including a significantly greater number of transitions with hyperfine splitting. More recent references and line lists for the crucial molecules, CO and OH, as well as for C
2
and SiH, are also included. In contrast to DR14, DR16 contains measurable lines from the heavy neutron-capture elements cerium (as Ce
ii
), neodymium (as Nd
ii
), and ytterbium (as Yb
ii
), as well as one line from rubidium (as Rb
i
), which may be detectable in a small fraction of APOGEE red giants.
ABSTRACT
G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star ...cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R = 15 000) spectrum obtained with the high-resolution spectrograph on the Hobby–Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate-resolution (R ∼ 4000) spectrum of the Ca ii triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium’s 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with $\rm {Fe/H}~=~-0.98\pm 0.05$. G1 also shows signs of α-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs – which suggests that G1’s formation is intimately connected with GC formation. G1’s high Na/Fe also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1’s chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.
Abstract
The measurement of the structure of stellar populations in the Milky Way disc places fundamental constraints on models of galaxy formation and evolution. Previously, the disc's structure has ...been studied in terms of populations defined geometrically and/or chemically, but a decomposition based on stellar ages provides a more direct connection to the history of the disc, and stronger constraint on theory. Here, we use positions, abundances and ages for 31 244 red giant branch stars from the Sloan Digital Sky Survey (SDSS)-APOGEE survey, spanning 3 < R
gc < 15 kpc, to dissect the disc into mono-age and mono-Fe/H populations at low and high
$\mathrm{ \alpha \mathrm{/Fe}}$
. For each population, with Δage < 2 Gyr and ΔFe/H < 0.1 dex, we measure the structure and surface-mass density contribution. We find that low
$\mathrm{ \alpha \mathrm{/Fe}}$
mono-age populations are fit well by a broken exponential, which increases to a peak radius and decreases thereafter. We show that this profile becomes broader with age, interpreted here as a new signal of disc heating and radial migration. High
$\mathrm{ \alpha \mathrm{/Fe}}$
populations are well fit as single exponentials within the radial range considered, with an average scalelength of 1.9 ± 0.1 kpc. We find that the relative contribution of high to low
$\mathrm{ \alpha \mathrm{/Fe}}$
populations at R
0 is
$f_\Sigma = 18\hbox{ per cent} \pm 5\hbox{ per cent}$
; high
$\mathrm{ \alpha \mathrm{/Fe}}$
contributes most of the mass at old ages, and low
$\mathrm{ \alpha \mathrm{/Fe}}$
at young ages. The low and high
$\mathrm{ \alpha \mathrm{/Fe}}$
populations overlap in age at intermediate Fe/H, although both contribute mass at R
0 across the full range of Fe/H. The mass-weighted scaleheight hZ
distribution is a smoothly declining exponential function. High
$\mathrm{ \alpha \mathrm{/Fe}}$
populations are thicker than low
$\mathrm{ \alpha \mathrm{/Fe}}$
, and the average hZ
increases steadily with age, between 200 and 600 pc.
ABSTRACT We present new identifications of five red giant stars in the Galactic halo with chemical abundance patterns that indicate they originally formed in globular clusters. Using data from the ...Apache Point Observatory Galactic Evolution Experiment (APOGEE) Survey available through Sloan Digital Sky Survey (SDSS) Data Release 12, we first identify likely halo giants, and then search those for the well-known chemical tags associated with globular clusters, specifically enrichment in nitrogen and aluminum. We find that 2% of the halo giants in our sample have this chemical signature, in agreement with previous results. Following the interpretation in our previous work on this topic, this would imply that at least 13% of halo stars originally formed in globular clusters. Recent developments in the theoretical understanding of globular cluster formation raise questions about that interpretation, and we concede the possibility that these migrants represent a small fraction of the halo field. There are roughly as many stars with the chemical tags of globular clusters in the halo field as there are in globular clusters, whether or not they are accompanied by a much larger chemically untaggable population of former globular cluster stars.
We present carbon abundances of red giants in Milky Way (MW) globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the ...clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of C/Fe. The C/Fe ratio in the clusters decreases with increasing luminosity above log(L/L sub(middot in circle)) Asymptotically = to 1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial C/Fe of the dSph giants is not uniform. Stars in dSphs at lower metallicities have larger C/Fe ratios. We hypothesize that C/Fe (corrected to the initial carbon abundance) declines with increasing Fe/H due to the metallicity dependence of the carbon yield of asymptotic giant branch stars and due to the increasing importance of SNe Ia at higher metallicities. We also identified 11 very carbon-rich giants (eight previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the MW stellar halo. Nonetheless, the stars with C/Fe < + 1 in dSphs follow a different C/Fe track with Fe/H than the halo stars. Specifically, C/Fe in dSphs begins to decline at lower Fe/H than in the halo. The difference in the metallicity of the C/Fe "knee" adds to the evidence from alpha /Fe distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs.
Abstract
We present analyses of improved photometric and spectroscopic observations for two detached eclipsing binaries at the turnoff of the open cluster NGC 752: the 1.01 days binary DS And and the ...15.53 days BD +37 410. For DS And, we find
M
1
= 1.692 ± 0.004 ± 0.010
M
⊙
,
R
1
= 2.185 ± 0.004 ± 0.008
R
⊙
,
M
2
= 1.184 ± 0.001 ± 0.003
M
⊙
, and
R
2
= 1.200 ± 0.003 ± 0.005
R
⊙
. We either confirm or newly identify unusual characteristics of both stars in the binary: the primary star is found to be slightly hotter than the main-sequence turnoff and there is a more substantial discrepancy in its luminosity compared to models (model luminosities are too large by about 40%), while the secondary star is oversized and cooler compared to other main-sequence stars in the same cluster. The evidence points to nonstandard evolution for both stars, but most plausible paths cannot explain the low luminosity of the primary star. BD +37 410 only has one eclipse per cycle, but extensive spectroscopic observations and the Transiting Exoplanet Survey Satellite light curve constrain the stellar masses well:
M
1
= 1.717 ± 0.011
M
⊙
and
M
2
= 1.175 ± 0.005
M
⊙
. The radius of the main-sequence primary star near 2.9
R
⊙
definitively requires large convective core overshooting (>0.2 pressure scale heights) in models for its mass, and multiple lines of evidence point toward an age of 1.61 ± 0.03 ± 0.05 Gyr (statistical and systematic uncertainties). Because NGC 752 is currently undergoing the transition from nondegenerate to degenerate He ignition of its red clump stars, BD +37 410 A directly constrains the star mass where this transition occurs.
To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the ...literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the alpha/Fe ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low alpha/Fe ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V
The open cluster NGC 6791 is among the oldest, most massive, and metal-rich open clusters in the Galaxy. High-resolution H-band spectra from the Apache Point Observatory Galactic Evolution Experiment ...(APOGEE) of 11 red giants in NGC 6791 are analyzed for their chemical abundances of iron, oxygen, and sodium. The abundances of these three elements are found to be homogeneous (with abundance dispersions at the level of ~0.05-0.07 dex) in these cluster red giants, which span much of the red-giant branch (T sub(eff) ~ 3500-4600 K), and include two red clump giants. From the infrared spectra, this cluster is confirmed to be among the most metal-rich clusters in the Galaxy (left angle bracketFe/Hright angle bracket = 0.34 0.06) and is found to have a roughly solar value of O/Fe and slightly enhanced Na/Fe, Our non-LTE calculations for the studied Na I lines in the APOGEE spectral region (16373.86 Angstrom and 16388.85 Angstrom) indicate only small departures from LTE (< or =, slant0.04 dex) for the parameter range and metallicity of the studied stars. The previously reported double population of cluster members with different Na abundances is not found among the studied sample.
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